In a manufacturing process of a flat panel display, a solar cell, a semiconductor device or the like, plasma is used to form a thin film or to perform an etching. By way of example, plasma is generated by introducing a gas into a vacuum chamber and applying a high frequency power having a band ranging from several MHz to several hundreds of MHz to an electrode provided within the chamber. In order to increase productivity, the size of a glass substrate for a flat panel display or a solar cell is getting larger year by year, and a glass substrate having a size exceeding about 2 meter square has already been used.
In a film forming process such as plasma CVD (Chemical Vapor Deposition), plasma having a higher density is required to increase a film forming rate. Further, plasma having a low electron temperature is also required in order to reduce ion irradiation damage by lowering the energy of ions incident on a surface of a substrate and in order to suppress excessive dissociation of gas molecules. In general, if a plasma excitation frequency is increased, a plasma density may increase whereas an electron temperature decreases. Accordingly, it may be desirable to set the plasma excitation frequency to be high in order to form a high-quality thin film with a high throughput. Thus, a high frequency in a VHF (Very High Frequency) band ranging from about 30 MHz to about 300 MHz, higher than about 13.56 MHz of a typically used high frequency power supply, has been used in a plasma process (see, for example, Patent Documents 1 and 2).
Patent Document 1: Japanese Patent Laid-open Publication No. H09-312268
Patent Document 2: Japanese Patent Laid-open Publication No. 2009-021256